Depth-varying light fields for three dimensional sensing

ABSTRACT

A method for mapping includes projecting onto an object a pattern of multiple spots having respective positions and shapes, such that the positions of the spots in the pattern are uncorrelated, while the shapes share a common characteristic. An image of the spots on the object is captured and processed so as to derive a three-dimensional (3D) map of the object.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 60/785,187, filed Mar. 24, 2006 and of U.S. ProvisionalPatent Application 60/852,436, filed Oct. 16, 2006. This application isa continuation-in-part of PCT Patent Application PCT/IL2006/000335,filed Mar. 14, 2006, which claims the benefit of U.S. Provisional PatentApplication 60/724,903, filed Oct. 11, 2005. All of these relatedapplications are assigned to the assignee of the present patentapplication, and their disclosures are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to methods and systems formapping of three-dimensional (3D) objects, and specifically to 3Doptical ranging and mapping.

BACKGROUND OF THE INVENTION

When a coherent beam of light passes through a diffuser and is projectedonto a surface, a primary speckle pattern can be observed on thesurface. The primary speckle is caused by interference among differentcomponents of the diffused beam. The term “primary speckle” is used inthis sense in the present patent application, in distinction tosecondary speckle, which is caused by diffuse reflection of coherentlight from the rough surface of an object

Hart describes the use of a speckle pattern in a high-speed 3D imagingsystem, in Taiwanese Patent TW 527528 B and in U.S. patent applicationSer. No. 09/616,606, whose disclosures are incorporated herein byreference. The system includes a single-lens camera subsystem with anactive imaging element and CCD element, and a correlation processingsubsystem. The active imaging element can be a rotating aperture whichallows adjustable non-equilateral spacing between defocused images toachieve greater depth of field and higher sub-pixel displacementaccuracy. A speckle pattern is projected onto an object, and images ofthe resulting pattern are acquired from multiple angles. The images arelocally cross-correlated using an image correlation technique, and thesurface is resolved by using relative camera position information tocalculate the three-dimensional coordinates of each locally-correlatedregion.

Another speckle-based 3D imaging technique is described by Hunter etal., in U.S. Pat. No. 6,101,269, whose disclosure is incorporated hereinby reference. A random speckle pattern is projected upon a 3D surfaceand is imaged by a plurality of cameras to obtain a plurality oftwo-dimensional digital images. The two-dimensional images are processedto obtain a three-dimensional characterization of the surface.

Other methods for 3D mapping and ranging use structured or codedillumination. For example, Sazbon et al. describe a method of this sortfor range estimation in “Qualitative Real-Time Range Extraction forPreplanned Scene Partitioning Using Laser Beam Coding,” PatternRecognition Letters 26 (2005), pages 1772-1781, which is incorporatedherein by reference. A phase-only filter codes the laser beam into Mdifferent diffraction patterns, corresponding to M different rangesegments in the workspace. Thus, each plane in the illuminated scene isirradiated with the pattern corresponding to the range of the plane fromthe light source. A common camera can be used to capture images of thescene, which may be processed to determine the ranges of objects in thescene. The authors describe an iterative procedure for designing thephase-only filter based on the Gerchberg-Saxton algorithm.

SUMMARY OF THE INVENTION

Embodiments of the present invention that are described hereinbelowprovide methods and systems for 3D mapping and ranging using shaped spotillumination patterns. Such patterns comprise an array of bright spotshaving a controlled, identifiable shape. In some embodiments, therelative positions of the spots are uncorrelated (for example, in arandom or pseudo-random pattern, such as a speckle pattern), but thespots in any case share a similar, predefined shape characteristic. Incertain embodiments, the spots are elongated in a certain direction,which is common to all the spots in a given plane transverse to theillumination beam, but other spot shapes may alternatively be used.

In some embodiments, the spot shape characteristic changes with distancefrom the illumination source. This distance-varying shape characteristicmay be achieved by passing the illumination beam through one or moreoptical elements that are designed to superpose two optical constraints:one to split the beam into multiple spots, and another to create thedistance-varying shape. This superposition approach permits a richvariety of distance-varying patterns to be created simply and flexibly.As a result of the distance-varying shape characteristic, the shapes ofthe spots appearing on parts of the surface of an object that isilluminated by the pattern may be used to determine the range of thoseparts from the source.

In some embodiments, transverse shifts of parts of the pattern on thesurface, relative to a reference pattern at a known range, are used toreconstruct a 3D map of the surface. The combination of shape-basedranging and shift-based mapping can be used to create an accurate 3D mapcovering a large range of distance from the illumination source.

There is therefore provided, in accordance with an embodiment of thepresent invention, a method for mapping, including:

projecting onto an object a pattern of multiple spots having respectivepositions and shapes, such that the positions of the spots in thepattern are uncorrelated, while the shapes share a commoncharacteristic;

capturing an image of the spots on the object; and

processing the image so as to derive a three-dimensional (3D) map of theobject.

In one embodiment, the pattern of spots includes a random specklepattern.

In some embodiments, the common characteristic of the shapes varies as afunction of distance from a source of the pattern, and processing theimage includes analyzing the characteristic of the spots on a surface ofthe object in the image so as to determine the distance of the surfacefrom the source. In one embodiment, the spots share an elongate shape,which rotates as a function of distance from a source of the pattern,and analyzing the characteristic includes determining a direction of thespots on the surface of the object. Additionally or alternatively,processing the image includes finding respective offsets between thepattern on multiple areas of the object in the image and the pattern ina reference image, and using the offsets together with the distance toderive the 3D map. Finding the respective offsets may include choosingthe reference image from among a plurality of reference imagesresponsively to the distance of the surface from the source.

In another embodiment, the spots in the pattern have an elongate shape,which is aligned in a first direction, and processing the image includesfinding respective offsets in a second direction, perpendicular to thefirst direction, between the pattern on multiple areas of the object inthe image and the pattern in a reference image so as to derive the 3Dmap. Projecting the pattern of spots may include passing a beam ofcoherent light through a diffuser, wherein the beam has a profile at thediffuser that is elongated in the second direction.

In a disclosed embodiment, capturing the image includes capturing asuccession of images while the object is moving, and processing theimage includes tracking a movement of the object by processing thesuccession of the images. In one embodiment, the object is a part of ahuman body, and tracking the movement includes identifying a gesturemade by the part of the human body and providing an input to a computerapplication responsively to the gesture.

There is also provided, in accordance with an embodiment of the presentinvention, a method for imaging, including:

defining a first optical constraint such that application of the firstoptical constraint to a beam of light splits the beam into a pattern ofmultiple spots;

defining a second optical constraint such that application of the secondoptical constraint to the beam of light causes the beam to form a spothaving a shape characteristic that changes in a predefined manner as afunction of a distance along an axis of the beam;

designing at least one optical element so as to superpose the first andsecond optical constraints; and

directing a beam of light through the at least one optical element so asto project the pattern onto a surface such that the multiple spots inthe pattern have the shape characteristic.

In some embodiments, the at least one optical element includes a firstoptical element for splitting the beam into the pattern and a secondoptical element for applying the shape characteristic. In oneembodiment, the pattern includes a speckle pattern, and the firstoptical element includes a diffuser.

Alternatively or additionally, the at least one optical element includesa diffractive optical element (DOE). In one embodiment, the DOE includesat least one zone plate for imparting an elongate shape to the spots.The at least one zone plate may include a plurality of superposed zoneplates, having different, respective periods and angular orientations soas to cause the elongate shape of the spots to rotate as a function ofthe distance.

Further alternatively or additionally, the at least one optical elementincludes a refractive optical element.

Typically, the pattern defined by the first optical constraint has aduty cycle that is no greater than ¼.

In some embodiments, the pattern defines respective positions of thespots such that the positions are uncorrelated.

In a disclosed embodiment, the second optical constraint causes the spotto have an elongate shape, which rotates as a function of the distance.In another embodiment, the second optical constraint causes the spot tohave an annular shape.

In some embodiments, the method includes capturing an image of the spotson the surface, and processing the image so as to determine the distanceof the surface from the at least one optical element.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for mapping, including:

an illumination assembly, which is configured to project onto an objecta pattern of multiple spots having respective positions and shapes, suchthat the positions of the spots in the pattern are uncorrelated, whilethe shapes share a common characteristic;

an imaging assembly, which is configured to capture an image of thespots on the object; and

an image processor, which is coupled to process the image so as toderive a three-dimensional (3D) map of the object.

There is further provided, in accordance with an embodiment of thepresent invention, apparatus for imaging, including:

at least one optical element, which is designed so as to superpose firstand second optical constraints, such that application of the firstoptical constraint to a beam of light splits the beam into a pattern ofmultiple spots, and application of the second optical constraint to thebeam of light causes the beam to form a spot having a shapecharacteristic that changes in a predefined manner as a function of adistance along an axis of the beam; and

a light source, which is configured to direct a beam of light throughthe at least one optical element so as to project the pattern onto asurface such that the multiple spots in the pattern have the shapecharacteristic.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, pictorial illustration of a system for 3D rangingand mapping, in accordance with an embodiment of the present invention;

FIG. 2 is a schematic top view of a speckle imaging device, inaccordance with an embodiment of the present invention;

FIG. 3A is a schematic representation of a speckle pattern created byprojection of a laser beam through a randomizing optical element, inaccordance with an embodiment of the present invention;

FIG. 3B is a schematic representation of a light pattern created by adiffractive optical element (DOE), in accordance with an embodiment ofthe present invention;

FIGS. 3C and 3D are schematic representations of speckle patternscreated by projection of a laser beam through the optical elements ofFIGS. 3A and 3B, in accordance with an embodiment of the presentinvention;

FIG. 4 is a flow chart that schematically illustrates a method for 3Dranging and mapping, in accordance with an embodiment of the presentinvention;

FIGS. 5A-5P are schematic representations of a set of zone plates atdifferent rotation angles, which are used in creating a DOE, inaccordance with an embodiment of the present invention;

FIG. 5Q is a schematic, frontal view of a DOE created by superposing thezone plates of FIGS. 5A-5P, in accordance with an embodiment of thepresent invention;

FIG. 6A is a schematic representation of a speckle pattern created byprojection of a laser beam through a randomizing optical element, inaccordance with an embodiment of the present invention;

FIG. 6B is a schematic representation of a light pattern created by adiffractive optical element (DOE), in accordance with an embodiment ofthe present invention; and

FIGS. 6C and 6D are schematic representations of speckle patternscreated by projection of a laser beam through the optical elements ofFIGS. 6A and 6B, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS System Overview

FIG. 1 is a schematic, pictorial illustration of a system 20 for 3Dranging and mapping, in accordance with an embodiment of the presentinvention. System 20 comprises an imaging device 22, which generates andprojects a pattern of spots onto an object 28 and captures an image ofthe spot pattern appearing on the object. Details of the design andoperation of device 22 are shown in the figures that follow and aredescribed hereinbelow with reference thereto.

In some embodiments, the pattern of spots that is projected by imagingdevice 22 comprises a speckle pattern. In the context of the presentpatent application and in the claims, the term “speckle pattern” refersto a projected pattern of bright spots whose positions are uncorrelatedin planes transverse to the projection beam axis. The positions areuncorrelated in the sense that the auto-correlation of the positions ofthe speckles in the pattern as a function of transverse shift isinsignificant for any shift larger than the spot size. Random patterns,such as those created by primary laser speckle (as described above), areuncorrelated in this sense. Patterns created by human or computerdesign, such as pseudo-random and quasi-random patterns, may also beuncorrelated. Other aspects of the theory and operation of speckle-based3D mapping, which may be implemented in system 20, are described in PCTPatent Application PCT/IL2006/000335, filed Mar. 14, 2006, and in a PCTpatent application entitled, “Three-Dimensional Sensing Using SpecklePatterns,” filed Mar. 8, 2007. Both of these applications are assignedto the assignee of the present patent application, and their disclosuresare incorporated herein by reference.

In other embodiments, the spots may be arranged in a regular, non-randompattern, such as the type of pattern that may be created by passing theillumination beam through a Damman grating or a suitable lenslet array.

For good performance in the ranging and mapping applications that aredescribed below, it is advantageous that the spot pattern have a lowduty cycle, i.e., that the fraction of the area of the pattern withabove-average brightness be no greater than 1/e, and desirably less than¼ or even 1/10. The low duty cycle is advantageous in enhancing thesignal/noise ratio of spot shift detection for 3D mapping. It also helpsto avoid interference effects that may result when neighboring spotsoverlap.

Even when the positions of the spots (speckles) are uncorrelated, theshapes of the spots in the patterns that are used in embodiments of thepresent invention are not entirely random, as in conventional laserspeckle patterns, but rather have a common shape characteristic. Forexample, in some embodiments, as described hereinbelow, the spots areelongated along a certain axis. Additionally or alternatively, the spotsmay have other common shape characteristics, so long as the shapes arecontrolled, and changes in the shapes as a function of distance alongthe axis of the illumination beam are identifiable. The term “changes ofshape” in this context means changes other than the simple linearincrease in spot size that normally occurs with distance from theillumination source.

An image processor 24 processes image data generated by device 22 inorder to perform depth ranging and, optionally, 3D mapping of object 28.The term “ranging,” as used in the present patent application and in theclaims, refers to finding a coarse measure of distance from the imagingdevice to the object, while the term “3D map” refers to a set of 3Dcoordinates representing the surface of the object. The derivation ofsuch a map based on image data is referred to as “3D mapping” orequivalently, “3D reconstruction.” Both ranging and mapping may be usedtogether, as coarse and fine phases, in the process of 3Dreconstruction, as described hereinbelow. Therefore, ranging may also beconsidered to be a sort of rough 3D mapping.

Image processor 24, which performs such ranging and mapping, maycomprise a general-purpose computer processor, which is programmed insoftware to carry out the functions described hereinbelow. The softwaremay be downloaded to processor 24 in electronic form, over a network,for example, or it may alternatively be provided on tangible media, suchas optical, magnetic, or electronic memory media. Alternatively oradditionally, some or all of the functions of the image processor may beimplemented in dedicated hardware, such as a custom or semi-customintegrated circuit or a programmable digital signal processor (DSP).Although processor 24 is shown in FIG. 1, by way of example, as aseparate unit from imaging device 22, some or all of the processingfunctions of processor 24 may be performed by suitable dedicatedcircuitry within the housing of the imaging device or otherwiseassociated with the imaging device.

The 3D map that is generated by processor 24 may be used for a widerange of different purposes. For example, the map may be sent to anoutput device, such as a display 26, which shows a pseudo-3D image ofthe object. In the example shown in FIG. 1, object 28 comprises all or apart (such as a hand) of the body of a subject. In this case, system 20may be used to provide a gesture-based user interface, in which usermovements detected by means of device 22 control an interactive computerapplication, such as a game, in place of tactile interface elements suchas a mouse, joystick or other accessory. Alternatively, system 20 may beused to create 3D maps of objects of other types, for substantially anyapplication in which 3D coordinate profiles are needed.

FIG. 2 is a schematic top view of device 22, in accordance with anembodiment of the present invention. An illumination assembly 30 indevice 22 comprises a coherent light source 34, typically a laser, andone or more optical elements 36, 38, which are typically used incombination to create a speckle pattern or other pattern of spots, asdescribed hereinbelow. (The term “light” in the context of the presentpatent application refers to any sort of optical radiation, includinginfrared and ultraviolet, as well as visible light.) The beam of lightemitted by source 34 passes through optical elements 36 and 38 andilluminates a target region 46 in which object 28 is located. An imagecapture assembly 32 captures an image of the pattern that is projectedonto object 28. Assembly 32 comprises objective optics 40, which focusthe image onto an image sensor 42. Typically, sensor 40 comprises arectilinear array of detector elements 44, such as a CCD or CMOS-basedimage sensor array.

In the embodiment shown in FIG. 2, illumination assembly 30 and imagecapture assembly 32 are held in a fixed spatial relation. Thisconfiguration and the processing techniques described hereinbelow makeit possible to perform 3D mapping using the single image captureassembly, without relative movement between the illumination and imagecapture assemblies and without moving parts. Alternatively, thetechniques of illumination, ranging and mapping that are describedhereinbelow may be used in conjunction with other sorts of image captureassemblies, in various different configurations, such as those describedin the Background section above. For example, the image capture assemblymay be movable relative to the illumination assembly. Additionally oralternatively, two or more image capture assemblies may be used tocapture images of object 28 from different angles.

To simplify the computation of the 3D map and of changes in the map dueto motion of object 28 in the configuration of FIG. 2, assemblies 30 and32 may be mounted so that an axis passing through the centers of theentrance pupil of image capture assembly 32 and the spot formed by lightsource 34 on optical element 36 is parallel to one of the axes of sensor40 (taken for convenience to be the X-axis, while the Z-axis correspondsto distance from device 22). The advantages of this arrangement areexplained further in the above-mentioned PCT patent applicationentitled, “Three-Dimensional Sensing Using Speckle Patterns.”

Specifically, by triangulation in this arrangement, a Z-direction shiftof a point on the object, δZ, will engender a concomitant transverseshift δX in the spot pattern observed in the image. Z-coordinates ofpoints on the object, as well as shifts in the Z-coordinates over time,may thus be determined by measuring shifts in the X-coordinates of thespots in the image captured by assembly 32 relative to a reference imagetaken at a known distance Z. Y-direction shifts may be disregarded. Thissort of triangulation approach is appropriate particularly in 3D mappingusing speckle patterns, although aspects of the approach may be adaptedfor use with other types of spot patterns, as well.

In other words, the group of spots in each area of the captured image iscompared to the reference image to find the most closely-matching groupof spots in the reference image. The relative shift between the matchinggroups of spots in the image gives the Z-direction shift of the area ofthe captured image relative to the reference image. The shift in thespot pattern may be measured using image correlation or other imagematching computation methods that are known in the art. Some exemplarymethods are described in the above-mentioned PCT patent applications.

Generation of Speckle Patterns With Common Shape Characteristics

Patterns of spots with a common shape characteristics can be used toenhance the operation of system 20 in a number of ways. For example, inthe above-mentioned configuration of device 22, in which onlyX-direction shifts are significant in 3D reconstruction of the object,it may be advantageous to use a pattern of speckles that are elongatedin the Y-direction. As a result, when processor 24 computes thecorrelation between images for the purpose of detecting Z-directionshifts, the computation will be insensitive to small shifts of thespeckles in the Y-direction. This feature improves the robustness of theX-direction shift computation (and may make it possible to use a smallercorrelation window in the computation).

A number of methods may be used to generate this sort of pattern ofspeckles, which are elongated in the Y-direction and whose positions inthe X-direction are uncorrelated. One possibility is to configureoptical element 36 as a diffuser, with a randomly-arranged array ofgrains that are elongated in the X-direction. (Element 38 may not berequired in this case.) The grains may be opaque, for example, or theymay alternatively be of different thickness so as to cause phase changesin the transmitted light. As another alternative, element 36 maycomprise a conventional, isotropic diffuser, while the beam from lightsource 34 is elongated in the X-direction. A cylindrical lens (notshown) between the source and diffuser may be used for this purpose, forexample.

Yet another option is that element 36 comprise a diffuser (which may beisotropic) and element 38 comprise a diffractive optical element (DOE).To generate the desired speckles with elongation along the Y-axis,element 38 may simply comprise a suitable grating or zone plate withlines oriented parallel to the X-axis.

Although elements 36 and 38 are shown in FIG. 2, for the sake ofconceptual clarity, as separate components, the diffuser and DOE aretypically in contact with one another and may be glued together.Alternatively, elements 36 and 38 may be made as a single piece, byforming the DOE on one side of an optical blank and grinding the otherside to create the diffuser. Further alternatively, the opticalconstraints imposed by the diffuser and grating or zone plate may becombined in a single DOE, in which the X-oriented lines and apseudo-random diffusing pattern are superposed, for example. In all ofthese cases, the pattern of speckles in the far field will be aconvolution of the random position distribution provided by the diffuserwith the shape defined by the Fourier transform of the DOE.

A similar superposition approach may be used in generating DOEs thatcreate more complex patterns, which vary with distance Z. In someembodiments, the DOE may be designed to create a pattern of spots havingdifferent shapes in different distance ranges. Referring to FIG. 2, thespots may have one shape at distance Z₁, another at Z₂, and so forth.For example, the elongated shape of the spots may be rotated by 45° fromrange to range. As a result, when object 28 is located at Z_(A), thespots will have the orientation of the range of Z₂, whereas when theobject is at Z_(B), the spots will have the orientation of the range ofZ₃. By detecting the orientation of the spots, processor 24 (FIG. 1) maythen determine the distance range of the object, independently of the 3Dreconstruction process based on spot position correlation. Ranging andmapping may be used in combination to generate 3D maps with enhancedrange and/or resolution, as described further hereinbelow.

Various methods may be used to design a single optical element thatcreates a pattern of spots with the desired Z-dependence. One methodthat may be used to design optical element 38 so as to give this sort ofrange-dependent speckle shaping is to use the iterative Gerchberg-Saxtonmethod, as described in the above-mentioned article by Sazbon et al. Theapproach, however, is computationally costly and is not guaranteed toconverge.

Alternatively, a set of one or more optical elements for such purposesmay be designed based on a superposition of constraints. For example,such an element or elements for generating speckles with four differentorientations in different, respective ranges may be produced bysuperposing a splitter (which generates a pattern of spots with low dutycycle and uncorrelated positions) with an element that, taken on itsown, implements a single Z-varying pattern at four orientations in fourpredefined ranges. The element creating the single pattern may be, forinstance, a superposition of four zone plates, each of which focuseslight to a line in the proper, respective direction in one of theranges.

Alternatively or additionally, the superposition may be computed andthen implemented in a suitable computer-generated hologram (CGH) orother DOE. Given a set of constraints R_(i), i=1, 2, . . . , K that areto be imposed on the light intensity (or equivalently, amplitude) atdistances Z_(i), i=1, 2, . . . , K, respectively, the optical elementsthat are to implement these constraints can be decomposed into aconvolution of two patterns, R_(i)=S

P_(i), wherein S is the same for R₁, R₂, . . . , R_(K), while P_(i) isallowed to vary arbitrarily with i as i=1, 2, . . . , K. The constraintscan be implemented using two filters F_(S), F_(v), corresponding tooptical elements 36 and 38, as follows:

-   -   Filter F_(S) creates intensity pattern S in the far field,        meaning it is an appropriate inverse Fourier transform of S        (which can be computed using techniques of DOE design that are        known in the art).    -   Filter F_(v) creates intensity patterns P_(i), i=1, 2, . . . , K        at distances Z_(i), i=1, 2, . . . , K , respectively. Intensity        patterns of this sort are generally much simpler to compute than        the above-mentioned patterns R_(i), i=1, 2, . . . , K, and may        generally be created individually using methods known in the        art.        The combined filter F=F_(S)·F_(v) (wherein the multiplication is        performed pointwise in the filter plane) will implement        intensity patterns R_(i), i=1, 2, . . . , K at distances Z_(i),        i=1, 2, . . . , K, respectively.

One shortcoming of this approach is that it is valid only for sets ofconstraints that can be represented as the sort of convolution R_(i)=S

P_(i). This shortcoming can be alleviated by using the linearity of theFresnel-Sommerfeld equations to decompose any set of constraints into asum of convolutions. A filter for each such convolution can then becreated by the technique described above, and the filters superposed togive the desired result.

FIGS. 3A-3D are schematic representations of images created byprojecting a laser beam through DOEs 36 and 38 that are designed inaccordance with an embodiment of the present invention. The object ofthis design is to create a pattern of elongated speckles whose axis ofelongation varies over 180° as a function of Z over a given range fromZ₀ to Z₀+ΔZ. FIGS. 3A and 3B show the patterns created by F_(S) andF_(v), respectively, which are defined as follows:

-   -   F_(S) creates a far-field pattern of random speckles, which need        not vary in Z. For example, F_(S) may be designed to create a        pattern of 1000×1000 pixels, comprising 200×200 bright speckles        50, randomly distributed, with the rest of the field dark.        Depending on the sizes of the spots, the duty cycle is typically        roughly between 4% and 25%. Optical element 36 to implement this        pattern may comprise a phase-only DOE, created using techniques        that are known in the art.    -   F_(v) creates N light intensity patterns in the near-field        volume between the planes Z₀ and Z₀+ΔZ. FIG. 3B shows one such        pattern at a given

$Z = {Z_{0} + {\frac{p}{N}\Delta\;{Z.}}}$ The pattern comprises V×W pixels, wherein V and W are typicallysmall—in this case V=W=5. For each p, p=1, . . . , N, the pattern isrotated by an angle θ=180*p/N relative to the vertical. Optical element38 implementing this pattern may be designed using the Gerchberg-Saxtonmethod described above, by superposing multiple zone plates as describedbelow, or using any other suitable method of DOE design that is known inthe art. Because the pattern is small, the computation required togenerate the DOE is relatively simple in any case.

FIGS. 3C and 3D show the pattern of speckles that is created bysuperposing elements 36 and 38, corresponding to F_(S) and F_(v), at twodifferent values of Z. The patterns comprise 200×200 elongated speckles52 and 54, respectively, oriented at different angles depending on Z. Asthe distance increases from Z₀ to Z₀+ΔZ, the speckles rotate by 180°. Bydetecting the angle θ of the speckles that are projected onto a surfaceof object 28, processor 24 is thus able to determine p=Nθ/180, giving

$Z = {Z_{0} + {\frac{p}{N}\Delta\; Z}}$ as the range of the surface from device 22.

FIG. 4 is a flow chart that schematically illustrates a method for 3Dranging and mapping, in accordance with an embodiment of the presentinvention. The method is described hereinbelow, for the sake of clarity,with reference to system 20, as illustrated in FIGS. 1 and 2, using thespeckle pattern shown in FIGS. 3C and 3D. This method may similarly beapplied, however, in speckle-based imaging systems of other sorts, suchas those described in the references cited above. Furthermore, othersorts of spot patterns and of Z-varying spot shape characteristics mayalternatively be created and used in place of the rotating linear shapeof the speckles shown in FIGS. 3C and 3D.

In preparation for ranging and imaging of an object, imaging device 22is operated to capture one or more reference speckle images. For thispurpose, for example, a planar surface may be placed at one or moreknown fiducial distances from the origin along the Z-axis, such as atZ₁, Z₂, Z₃, . . . . Imaging assembly 32 captures a reference image ofthe speckle pattern that is projected onto the surface by illuminationassembly 30 at each distance. Alternatively, since the speckle patternis essentially a convolution of the small Z-varying pattern of F_(v) andthe far-field spot pattern of F_(S), respective reference patterns canbe determined separately for the two filters and then convolved to givethe combined pattern at each distance Z. This approach can reduce theamount of memory required for storing the reference patterns. Furtheralternatively, the design of the entire pattern can be standardized,thus rendering the capture of the reference unnecessary.

Object 28 is introduced into target region 46, and device 22 captures atest image of the speckle pattern that is projected onto the surface ofthe object, at a test capture step 60. Processor 24 then computes theorientation angle of the speckles, at a ranging step 62. For thispurpose, the processor may, for example, perform a spectral analysis ofthe test image. The shape of the spectrum (for example, the directionsof the major and minor axes) will correspond to the orientation angle ofthe speckles, which will in turn indicate the range of the object.Alternatively or additionally, the processor may compute thecross-correlation between the speckle shape and a number of differentreference shapes. Of course, if the object is sufficiently deep, thespeckle angle may vary over the surface, so that different parts of theobject will have different ranges. Processor 24 may identify the loci ofany abrupt changes in speckle angle as edges of the object.

Processor 24 may use the shape-based range measurement of step 62 byitself in generating a rough 3D map of object 28. In the embodimentshown in FIG. 4, however, the processor uses this range measurement inconjunction with spot position triangulation in reconstructing the 3Dmap of object 28, at a reconstruction step 64. For this purpose, theprocessor typically measures local offsets between the speckle patternat different points on the object surface in the test image andcorresponding areas of the speckle pattern in the appropriate referenceimage. The processor then uses triangulation, based on the offsets, todetermine the Z-coordinates of these object points. Methods that may beused for these purposes are described in the above-mentioned PCT patentapplications and in the other references cited above.

The combination of ranging at step 62 with 3D reconstruction at step 64enables system 20 to perform 3D reconstruction with greater precisionand/or lower computational cost than can generally be achieved byspeckle-based triangulation alone, and may also be used to increase therange in the Z-direction over which the measurement is made. Forexample, if multiple reference images were captured at differentfiducial distances, as mentioned above, the processor can measure thelocal offsets relative to the reference image whose fiducial distance isclosest to the range found at step 62. Even if only a single referenceimage is used, the triangulation accuracy and/or speed at step 64 can beenhanced since processor 24 can use the ranging result to limit thenumber of different local offset values that it has to check or tocompute the offset with greater precision.

Steps 60-64 may be repeated continually in order to track motion ofobject 28 within target region 46. For this purpose, device 22 capturesa succession of test images while the object is moving, and processor 24repeats steps 62 and 64 in order to track the 3D movement of the object.It may be possible to skip step 62 in some iterations by assuming thatthe object has not moved too far since the previous iteration.

In an alternative embodiment (not shown in the figures), triangulationbased on shifts of the speckle pattern is used for coarse rangemeasurement, and changes in the speckle shape characteristic are usedfor accurate 3D mapping. The accuracy of triangulation depends, interalia, on the separation along the X-axis between illumination assembly30 and image capture assembly 32. If only coarse triangulation isrequired, assemblies 30 and 32 can be positioned close together, thuspermitting a more compact design of device 22.

Enhanced accuracy of the shape-based Z-direction measurement may beachieved, for example, by replicating the Z-varying pattern of thespeckle shape over several cycles within target region 46. In otherwords, taking the example of the rotating linear speckle shape describedabove and the arrangement shown in FIG. 2, the speckle orientation mayvary over 180° between Z₁ and Z₂, and then again between Z₂ and Z₃, andagain between Z₃ and Z₄. Processor 24 uses the speckle triangulationresult to decide in which of these three ranges object 28 is located,and then uses the speckle orientations to construct the precise 3D mapof the object within this range. Alternatively, as noted earlier, otherZ-varying spot shape characteristics may be used in this context inplace of the directional variation illustrated in FIGS. 3C and 3D.

Various methods may be used to replicate the Z-varying shape overmultiple cycles in Z. For example, the pattern may be focused ontotarget region 46 using a suitable multifocal lens (not shown). Such alens may comprise, for example, a superposition of several Fresnel zoneplates with different respective focal lengths. Alternatively, asuitable multifocal lens may be designed using techniques described byBen Eliezer, et al., in “All Optical Extended Depth of Field ImagingSystem,” Journal of Optica and Pure Physics—A, 5 (2003), pagesS164-S169, which is incorporated herein by reference. Furtheralternatively, optical element 38 may be designed ab initio, using thesuperposition techniques described above, to give a speckle shape thatrepeats over multiple cycles.

FIGS. 5A-5Q schematically illustrate a method for producing opticalelement 38 so as to create a pattern of shaped spots, in accordance withan embodiment of the present invention. FIGS. 5A-5P are schematic,frontal views of a set of zone plates 70, while FIG. 5Q is a schematicfrontal view of a DOE 72 created by superposing the zone plates of FIGS.5A-5P. DOE 72 can serve as the filter F_(v), for use together with asuitable diffuser or other beam splitting filter F_(S), in order toproduce the sort of rotating, linear spots that are shown above in FIGS.3C and 3D.

Each of zone plates 70 is a Fresnel zone plate of focal length f_(n),n=1, . . . , N (wherein N=16 in this example), designed for wavelengthλ. The transmission function of such a zone plate is given by:

${f( {x,y} )} = \{ \begin{matrix}1 & {{{if}\mspace{14mu}{\cos( \frac{\pi\; x^{2}}{\lambda\; f} )}} > 0} \\0 & {otherwise}\end{matrix} $wherein the X-Y axes in each zone plate are rotated by (n−1)π/N.Alternatively, zone plates 70 may be designed as transparent, phase-onlyzone plates. DOE 72 comprises a superposition 74 of these N zone plates.This superposition will produce a line which rotates as a function of Zat a rate of π/N(f_(n)−f_(n−1)) rad/m.

Various methods may be used to superpose zone plates 70. For example,the superposition may be produced, as shown in FIG. 5Q, as an assemblageof “pie slices” cut from the sixteen component zone plates.Alternatively, pieces of the different zone plates may be extracted atrandom and assembled to create the superposition. Further alternatively,the component zone plates may be summed at each point in DOE 72 tocreate a single pattern, with appropriate quantization of the summedtransmittance or phase shift at each point.

Alternatively, optical element 38 may comprise an array of refractivelenses instead of the zone plates described above. In this case, asuperposition of cylindrical lenses (such as micro-lenses) at differentorientations, in pie-slice or random distribution, may be used to createthe desired Z-varying pattern.

FIGS. 6A-6D are schematic representations of images created byprojecting a laser beam through DOEs that are designed in accordancewith another embodiment of the present invention. Although the examplesshown in the preceding embodiments relate specifically to patterns ofspeckles with rotating linear shapes, the principles of the presentinvention may similarly be applied in creating Z-varying patterns ofother types and in optical ranging using such patterns. FIGS. 6A-6D showone example, in which speckles 80 and 82 have a common morphologicalcharacteristic, in the form of a circular or elliptical shape. Theradius of the shape of each spot varies with distance from theillumination assembly at a different rate from the linear magnificationof the pattern as a whole, thereby providing information on thedistance. Other sorts of shapes and patterns will be apparent to thoseskilled in the art and are considered to be within the scope of thepresent invention.

FIGS. 6A and 6B show the patterns created by F_(S) and F_(v),respectively. As in the preceding examples, F_(S) creates a pattern ofrandom speckles 50, which need not vary in Z. F_(v) creates an annularlight intensity pattern in the target region. The radius of the annulusvaries with Z in such a fashion as to enable ranging based on the radiiof the spots that are observed in the image captured by image captureassembly 32. As explained above, an optical element implementing thispattern may be designed using the Gerchberg-Saxton method, or bysuperposing multiple diffractive optical elements that create simplecircular or elliptical patterns with different size characteristics, orusing any other suitable method of DOE design that is known in the art.

FIGS. 6C and 6D show images of the pattern of speckles that is projectedonto a surface by passing coherent light through elements 36 and 38(corresponding to F_(S) and F_(v)), with the surface at two differentvalues of Z. The images are shown as they would be captured by imagecapture assembly 32. The patterns comprise elliptical speckles 80 and82, respectively, with different radii depending on Z. As the distanceincreases from Z₀ to Z₀+ΔZ, the radii of the speckles that are projectedonto the surface of object 28 and imaged onto sensor 42 increase ordecrease at a known rate. By measuring the radii of the speckles,processor 24 is thus able to determine the range of the surface fromdevice 22. Furthermore, the processor may detect and analyze changes inthe shapes of the speckles in order to determine the angular skew of thesurface relative to the X-Y plane.

Although the embodiments described above relate specifically tosuperposition of two types of patterns, which are typically implementedusing two optical elements 36 and 38, the design approach that is usedin this embodiments may be extended to superposition of three or morepatterns. The approach is applicable, as noted above, to a constraint ion the projected light pattern that can be presented as a convolution ofL patterns R_(i)=P_(1i)

P_(2i)

. . .

P_(Li) In this case, L filters F₁, 1=1, 2, . . . , L are designed suchthat filter F₁ implements patterns P_(1i), i=1, 2, . . . , K at Z_(i),i=1, 2, . . . , K respectively. The combined filter

$F = {\prod\limits_{{l = 1},2,\ldots\mspace{14mu},L}^{\;}\; F_{l}}$implements R_(i)=P_(1i)

P_(2i)

. . .

P_(Li) on a Z-scale that is reduced by a factor L. Therefore, at thedesign stage, the filters are scaled up by a factor L. It can be shown,using general scalar diffraction theory, that the field distribution atz, U(x, y, z), will be given by

${{U( {x,y,z} )} = {\underset{l = 1}{\overset{L}{\otimes}}{U_{l}( {x,y,z} )}}},$ wherein U₁(x, y, z) is the pattern filter 1 would create at distance z,and

$\underset{l = 1}{\overset{L}{\otimes}}$ denotes convolution.

Furthermore, although the embodiments described above relate tospecifically to speckle shaping and speckle-based 3D ranging andmapping, the methods described above for designing multi-constraintfilters and other optical elements—and particularly elements that createZ-varying patterns—may also be used to create optical elements in otherapplications in which complex light patterns are needed.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

1. A method for imaging, comprising: defining a first optical constraintsuch that application of the first optical constraint to a beam of lightsplits the beam into a pattern of multiple spots; defining a secondoptical constraint such that application of the second opticalconstraint to the beam of light causes the beam to form a spot having ashape characteristic that changes in a predefined manner as a functionof a distance along an axis of the beam; designing a first filter toimplement the first optical constraint and a second filter, differentfrom the first filter, to implement the second optical constraint;creating at least one optical element as a superposition of the firstand second filters; and directing a beam of light through the at leastone optical element so as to project the pattern onto a surface suchthat the multiple spots in the pattern have the shape characteristic. 2.The method according to claim 1, wherein the at least one opticalelement comprises a first optical element, comprising the first filter,for splitting the beam into the pattern and a second optical element,comprising the second filter, for applying the shape characteristic. 3.The method according to claim 2, wherein the pattern comprises a specklepattern.
 4. The method according to claim 3, wherein the first opticalelement comprises a diffuser.
 5. The method according to claim 1,wherein the at least one optical element comprises a diffractive opticalelement (DOE).
 6. The method according to claim 5, wherein the DOEcomprises at least one zone plate for imparting an elongate shape to thespots.
 7. The method according to claim 6, wherein the at least one zoneplate comprises a plurality of superposed zone plates, having different,respective periods and angular orientations so as to cause the elongateshape of the spots to rotate as a function of the distance.
 8. Themethod according to claim 1, wherein the at least one optical elementcomprises a refractive optical element.
 9. The method according to claim1, wherein the pattern defined by the first optical constraint has aduty cycle that is no greater than ¼.
 10. The method according to claim1, wherein the pattern defines respective positions of the spots suchthat the positions are uncorrelated.
 11. The method according to claim1, wherein the second optical constraint causes the spot to have anelongate shape, which rotates as a function of the distance.
 12. Themethod according to claim 1, wherein the second optical constraintcauses the spot to have an annular shape.
 13. The method according toclaim 1, and comprising capturing an image of the spots on the surface,and processing the image so as to determine the distance of the surfacefrom the at least one optical element.
 14. Apparatus for imaging,comprising: at least one optical element, which is designed as asuperposition of different, first and second filters, which respectivelyimplement first and second optical constraints, such that application ofthe first optical constraint to a beam of light splits the beam into apattern of multiple spots, and application of the second opticalconstraint to the beam of light causes the beam to form a spot having ashape characteristic that changes in a predefined manner as a functionof a distance along an axis of the beam; and a light source, which isconfigured to direct a beam of light through the at least one opticalelement so as to project the pattern onto a surface such that themultiple spots in the pattern have the shape characteristic.
 15. Theapparatus according to claim 14, wherein the at least one opticalelement comprises a first optical element, comprising the first filter,for splitting the beam into the pattern and a second optical element,comprising the second filter, for applying the shape characteristic. 16.The apparatus according to claim 15, wherein the pattern comprises aspeckle pattern.
 17. The apparatus according to claim 16, wherein thefirst optical element comprises a diffuser.
 18. The apparatus accordingto claim 14, wherein the at least one optical element comprises adiffractive optical element (DOE).
 19. The apparatus according to claim18, wherein the DOE comprises at least one zone plate for imparting anelongate shape to the spots.
 20. The apparatus according to claim 19,wherein the at least one zone plate comprises a plurality of superposedzone plates, having different, respective periods and angularorientations so as to cause the elongate shape of the spots to rotate asa function of the distance.
 21. The apparatus according to claim 14,wherein the at least one optical element comprises a refractive opticalelement.
 22. The apparatus according to claim 14, wherein the patterndefined by the first optical constraint has a duty cycle that is nogreater than ¼.
 23. The apparatus according to claim 14, wherein thepattern defines respective positions of the spots such that thepositions are uncorrelated.
 24. The apparatus according to claim 14,wherein the second optical constraint causes the spot to have anelongate shape, which rotates as a function of the distance.
 25. Theapparatus according to claim 14, wherein the second optical constraintcauses the spot to have an annular shape.
 26. The apparatus according toclaim 14, and comprising an imaging assembly, which is configured tocapture an image of the spots on the surface, and an image processor,which is coupled to process the image so as to determine the distance ofthe surface from the at least one optical element.
 27. A method forimaging, comprising: defining a target pattern of multiple spots as aconvolution of at least first and second patterns of different types;superposing at least first and second filters, which are designed tocreate the first and second patterns, respectively; and directing a beamof light through the superposed filters so as to project the targetpattern onto a surface.
 28. The method according to claim 27, whereinsuperposing at least the first and second filters comprises providing afirst optical element comprising the first filter and a second opticalelement comprising the second filter.
 29. The method according to claim27, wherein at least one of the first and second filters comprises adiffractive optical element.
 30. The method according to claim 27,wherein at least one of the first and second filters comprises multiplemicro-lenses.
 31. The method according to claim 27, and comprisingcapturing an image of the spots on the surface, and processing the imageso as to construct a three-dimensional map of the surface.
 32. Apparatusfor imaging, comprising: a first filter and a second filter, which aretogether configured to create, by superposition of the first and secondfilters, a target pattern of multiple spots, which is a convolution of afirst pattern created by the first filter and a second pattern, of adifferent type from the first pattern, created by the second filter; anda light source, which is configured to direct a beam of light throughthe first and second filters in superposition so as to project thetarget pattern onto a surface.
 33. The apparatus according to claim 32,and comprising a first optical element comprising the first filter and asecond optical element comprising the second filter.
 34. The apparatusaccording to claim 32, wherein at least one of the first and secondfilters comprises a diffractive optical element.
 35. The apparatusaccording to claim 32, wherein at least one of the first and secondfilters comprises multiple micro-lenses.
 36. The apparatus according toclaim 32, and comprising an imaging assembly, which is configured tocapture an image of the spots on the surface, and an image processor,which is coupled to process the image so as to construct athree-dimensional map of the surface.